Global Warming Potential (GWP)

GHGs warm the earth by absorbing energy and slowing the rate at which the energy escapes to space. Different GHGs can have different effects on the earth’s warming depending on their ability to absorb energy (radiative efficiency) and how long they stay in the atmosphere (lifetime).

The Global Warming Potential (GWP) was developed to allow comparisons of the global warming impacts of different gases. It is a measure of how much energy the emission of 1 Ton of a gas will absorb over a given period of time, relative to the emissions of 1 Ton of carbon dioxide. The larger the GWP, the more that given gas warms the earth compared to CO2 over that time period (usually 100 years). GWP provides a common unit of measure which allows analysts to add up emissions estimates of different gases (e.g., to compile a national GHG inventory), and allows policymakers to compare emissions reduction opportunities across sectors.

• CO2, by definition, has a GWP of 1 regardless of the time period used since it is the gas used as the reference. CO2 emissions cause an increase in the atmospheric concentrations that will least thousands of years.

• Methane (CH4) is estimated to have a GWP of 28-36 over 100 years. CH4 emitted today lasts about a decade on average, which is much less time than CO2. But CH4 absorbs much more energy than CO2.

• Nitrous Oxide (N2O) has a GWP 265-298 times that of CO2 for a 100-year timescale. N2O emitted today remains in the atmosphere for more than 100 years, on average.

• Fluorinated gases are high-GWP gases because, for a given amount of mass, they trap substantially more than CO2. The GWPs for these gases can be in the thousands or tens of thousands.

A direct interpretation is that the GWP is an index of the total energy added to the climate system by a component in question relative to that added by CO2.

However, the GWP does not lead to equivalence with temperature or other climate variables (Fuglestvedt et al., 2000, 2003; O’Neill, 2000; Daniel et al., 2012; Smith and Wigley, 2000; Tanaka et al., 2009). Thus, the name ‘Global Warming Potential’ may be somewhat misleading, and ‘relative cumulative forcing index’ would be more appropriate. It can be shown that the GWP is approximately equal to the ratio (normalizing by the similar expression for CO2) of the equilibrium temperature response due to a sustained emission of the species or to the integrated temperature response for a pulse emission (assuming efficacies are equal for the gases that are compared; O’Neill, 2000; Prather, 2002; Shine et al., 2005a; Peters et al., 2011a; Azar and Johansson, 2012).

The GWP has become the default metric for transferring emissions of different gases to a common scale; often called ‘CO2 equivalent emissions’ (e.g., Shine, 2009). It has usually been integrated over 20, 100 or 500 years consistent with Houghton et al. (1990). Note, however that Houghton et al. presented these time horizons as ‘candidates for discussion [that] should not be considered as having any special significance’. The GWP for a time horizon of 100 years was later adopted as a metric to implement the multi-gas approach embedded in the United Nations Framework Convention on Climate Change (UNFCCC) and made operational in the 1997 Kyoto Protocol. The choice of time horizon has a strong effect on the GWP values — and thus also on the calculated contributions of CO2 equivalent emissions by component, sector or nation. There is no scientific argument for selecting 100 years compared with other choices (Fuglestvedt et al., 2003; Shine, 2009).

The choice of time horizon is a value judgement because it depends on the relative weight assigned to effects at different times. Other important choices include the background atmosphere on which the GWP calculations are superimposed, and the way indirect effects and feedbacks are included. For some gases the variation in GWP with time horizon mainly reflects properties of the reference gas, not the gas for which the GWP is calculated. The GWP for NTCFs decreases with increasing time horizon, as GWP is defined with the integrated RF of CO2 in the denominator. As shown below, after about five decades the development in the GWP for CH4 is almost entirely determined by CO2. However, for longlived gases (e.g., SF6) the development in GWP is controlled by both the increasing integrals of RF from the long-lived gas and CO2.